1. Field of the Invention
The present invention relates to an optical information recording medium and its production, and more particularly, to a means for protecting the recording film or reflecting film formed on the substrate and preventing these films from corrosion.
2. Discussion of Prior Art
In recent years, for recording media which are of high recording density and excellent in random access characteristics, optical information recording media represented, for example, by compact disks, video disks, optical disk memories for computer, etc. are rapidly being used.
FIG. 7 is a perspective sectional view of a conventionally known compact disk. On one side of a disk-form substrate 11 is spirally formed a row of pits 12 which modulate information signals into a mechanical uneven form. To the surface of this row of pits 12 is formed a reflecting film 13 made of a metal such as for example aluminum, and on this metallic reflecting film 13 is formed a protecting film 14 for protecting the reflecting film 13.
The foregoing substrate 11 is made of a polycarbonate resin because it should have a high permeability to light, a good moldability and a small optical strain. Further, UV-curing resins are preferably used to form the protecting film 14 because of their high mass-productivity.
The surface of the substrate to be irradiated with light beams is exposed to the atmosphere, so that it is often easy to scratch and dust easily adheres to it. Also, the surface of the substrate is sometimes stained for example with solvents, by mistake. When the surface to be irradiated with light beams is scratched, incident light beams are randomly reflected at the surface to effect the level fluctuation of regenerated information signals. Also, when dust or a solvent adheres to the surface, the amount of transmitted light beams is changed to effect the level fluctuation of regenerated information signals. Particularly, when solvents adhere to the surface, the plastic substrate dissolves and, as a result, the surface to be irradiated with light beams is damaged. When dust or solvents have adhered to the surface, it suffices to wipe them off, but because the plastic substrate is relatively soft, the surface to be irradiated with light beams is easily scratched and, because the surface is getting charged, dust adheres more easily.
Both polycarbonate resins constituting the substrate 11 and UV-curing resins constituting the protecting film 14 are a resin material which is relatively low in permeability to moisture.
However, compact disks are sometimes exposed to a high-temperature and high-humidity atmosphere for a long time, for example, as in the case where they are used in automobiles. In this case, the foregoing resin material absorbs a fair amount of moisture. As a result, there is a problem that moisture absorbed by the substrate 11 and protecting film 14 reacts with the metallic reflecting film 13 to form a hydroxide, whereby the reflectance of laser beams for regeneration decreases so as to worsen the S/N ratio of regenerated signals.
The above explanation was made with reference to compact disks, but there is also the same problem in the case of other optical information recording media having a substrate and a protecting film, both of which are made of resins. As to optical information recording media with a glass substrate, there is no problem of moisture permeating the substrate. However, if measures against the permeation of moisture into the protecting film are not taken, the same problem as above likewise occurs.
For the purpose of protecting the surface to be irradiated with light beams from being scratched and the adhesion of dust, it has already been proposed to form a protective film made of a solvent-type UV-curing resin on the surface (JP-A-63-188837). Even this measure is not seen to be sufficient in terms of the prevention of moisture permeation.
Also, from the standpoint of the prevention of moisture permeation, there is proposed a method of covering the substrate surface of optical information recording media with fluorine-containing resins by the application of plasma polymerization (JP-A-60-147392). This method, however, is not sufficient in terms of corrosion of the recording or reflecting film because the permeation of moisture from the side of the protecting film is not satisfactorily prevented. Complete prevention of moisture permeation is therefore impossible by this constitution.